The major problem associated with the design and operation of loop detectors is due to the fact that the shift of oscillator frequency resulting from the passage of a vehicle over the loop is relatively small in comparison with, for instance, the long term frequency drift which is caused by changes in environmental conditions. Another problem is created by spurious interferences and cross-coupling between adjacent loop-sensors which results in transient frequency shifts. These problems are compounded by the aperiodical nature of the signal which imposes severe restrictions on the time constants of filters which might be used throughout the circuit to compensate for these problems.
Traffic signal controls, however, must be designed with a large margin of reliable operation, must be able to operate in very severe weather conditions and yet require a minimum of periodical maintenance and calibration.
Digital circuits have been used extensively during the last few years in the design of loop detectors in order to palliate the drift problems normally associated with analog circuitry. This preference for digital circuits has in most cases resulted in an increase in the number of necessary components. This, in turn, not only increases the cost of the device but also multiplies the chances of component failures.